1. Field of the Invention
The present invention is directed generally to a device including a flexible circuit disposed within a conductive tube and a method of making the device. The device may be any one of many devices, such as filters, directional couplers, power dividers, amplifiers, microwave mixers, and microwave fuses.
2. Description of the Background
Many products, such as cellular and PCS band telephones, require circuit devices, such as filters, directional couplers, power dividers, amplifiers, microwave mixers, and microwave fuses. Many new product designs require that the devices be smaller, less expensive, and perform to higher specifications than previously required, and it is increasingly difficult to obtain the desired performance from existing device designs. Lowpass filters will be discussed as an example of the difficulties faced with the prior art, although other types of devices may also benefit from the present invention. Lowpass filters pass signals below a predetermined cutoff frequency, and attenuate signals above the predetermined cutoff frequency. The attenuation of signals occurs in what is known as a stopband. Ideally, the stopband extends from the cutoff frequency to an infinite frequency. In practice, however, the stopband will end at some finite frequency above the cutoff frequency. The frequency range between the cutoff frequency and a point where the filter no longer meets rejection or attenuation requirements is known as the stopband width.
Present specifications for cellular and PCS band telephones require lowpass filter stopbands to be maintained up to about 12.5 GHz. Some prior art lowpass filter designs cannot be cost-effectively manufactured to those specifications, and other designs have undesirable characteristics. For example, traditional combline filters are bandpass filters and some higher frequency signals may pass through the filter with little loss because of higher order modes and harmonic multiples of the original passband. Most narrow band bandpass filters have stopbands that hold to slightly more than three times the center frequency of the passband. Wide band filters may hold to about five times the center frequency. One or more lowpass filters may be used in combination with a bandpass filter to increase stopband width, and such lowpass filters are usually referred to as "clean-up" lowpass filters. Although cascades of clean-up filters may be used to increase a stopband width to higher frequencies, clean-up filters do have parasitic passbands above their stopband.
Other alternatives include lumped element lowpass filters, but they are expensive, due in part to expensive substrate material and labor intensive assembly. Stripline lowpass filters require relatively expensive substrate material above and below the circuit, and grounding on both sides of the filter is critical. Furthermore, dielectric loading of inductive elements degrades the stopband width. Micro-stripline lowpass filters require expensive substrate material below the circuit and may have a nominally better stopband relative to stripline filters. However, dielectric loading of inductive elements reduce the high/low impedance ratio, which reduces performance. Modified micro-stripline lowpass filters may provide higher inductor impedance, but requires increased manual assembly and machining of parts, thereby increasing costs. Coaxial lowpass filters, also known as "bead and wire", generally provide the best lowpass filter performance. However, coaxial lowpass filters require machining and/or manual assembly, thereby increasing the cost of the device. Furthermore, the impedance ratio is limited by the power handling capability of the device to prevent inductor fusing.
Accordingly, there is a need for a new type of device that is inexpensive, easy to assemble, and that has good performance to better meet the demands of new and old technologies.